ABSTRACT- Airborne contaminants, including semi-volatile organic compounds (SOCs), have been shown to undergo atmospheric long range transport and deposition to high elevation and/or high latitude ecosystems. The Western Airborne Contaminant Assessment Program (WACAP) is aimed at evaluating the potential ecological impact from atmospheric transport and deposition of SOCs to high elevation ecosystems in National Parks located in Western North America. At high elevations within the parks, snow is the dominant form of precipitation and provides 50 to 90 percent of the deposition into high elevation and/or latitude ecosystems. SOCs are deposited by cold condensation and/or by snow either in the ice nuclei or scavenged from the air by the snowflakes. During the summer, the annual snow pack melts and delivers the SOCs into lakes. Lake water samples were collected from nine sites located in five national parks, three parks in Alaska and two parks in the contiguous United States. Multiple 50 liter lake water samples were collected from nine oligotrophic perched lakes during the ice free summer (four lakes sampled in 2003 and five lakes in 2004). Lake water samples were filtered and extracted in situ to determine the particulate versus dissolved phase concentrations. SOCs in the dissolved phase were extracted with a hydrophobic/hydrophilic solid phase extraction device while SOCs in the particulate phase were extracted from the glass fiber filter using accelerated solvent extraction. Extracts were analyzed for 84 SOCs including historic and current-use pesticides, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls using gas chromatographic mass spectrometry with electron impact and electron capture negative ionization, using isotope dilution and selective ion monitoring. Analyte ratios, including isomer ratios, current versus historic-use compound ratios, and parent versus metabolite ratios provide insight into possible SOC sources. Site differences, including elevation, latitude, and annual mean temperature, are examined with respect to the SOCs measured and their concentration.